Redux原理探索
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Redux
redux的index.js暴露以下几个接口
export {
createStore,
combineReducers,
bindActionCreators,
applyMiddleware,
compose,
__DO_NOT_USE__ActionTypes
}
先看createStore方法，下面是使用方法
const store = createStore(rootReducer)
下面为createStore的实现过程及返回值
//
初始化时设置一个默认的action
dispatch({ type: ActionTypes.INIT })
return
{
dispatch,
subscribe,
getState,
replaceReducer,
[$$observable]: observable
}
createStore返回了一些方法给store
- dispatch是唯一一个改变state的方法，用来更新state数据并触发监听
- subscribe订阅监听，返回一个方法解除监听
- getState返回state数据
- replaceReducer替换掉当前的reducer并初始化数据
dispatch在执行时先进行验证保证dispatch的参数action是有效的，然后执行下述代码
try
{
isDispatching = true//改变state
currentState = currentReducer(currentState, action)
} finally {
isDispatching = false
}
//触发监听
const listeners = (currentListeners = nextListeners)
for (let i = 0; i < listeners.length; i++) {
const listener = listeners[i]
listener()
}
return action
通过currentState = currentReducer(currentState, action)来改变state，其中currentReducer是初始化store时传入的参数rootReducer。
看看currentReducer是什么
export default combineReducers({ todos, visibilityFilter })
从使用方法上看是combineReducers接口的返回值
return
function combination(state = {}, action) {
let hasChanged = false
const nextState = {}
for (let i = 0; i < finalReducerKeys.length; i++) {
const key = finalReducerKeys[i]
const reducer = finalReducers[key]
const previousStateForKey = state[key]
const nextStateForKey = reducer(previousStateForKey, action)
if (typeof nextStateForKey === ‘undefined‘) {
const errorMessage = getUndefinedStateErrorMessage(key, action)
thrownew Error(errorMessage)
}
//将改变后的state存放到全局变量
nextState[key] = nextStateForKey
//判断state是否改变 只要有一个key变了就认为state改变了
hasChanged = hasChanged || nextStateForKey !== previousStateForKey
}
//根据是否改变选择返回旧state还是新的statereturn hasChanged ? nextState : state
}
返回的是combineReducers闭包函数，在返回之前先对reducer里面的方法进行验证保证方法好用
currentReducer(currentState, action)实际上就是combination(currentState, action)，与旧的state进行比较来确定是否更新
componentdidmount(){
const {changeCurrentMonday,changeCurrentMonth} = this.props;
changeCurrentMonday(data);
}
...
const mapDispatchToProps = (dispatch) => {
return bindActionCreators({
changeCurrentMonday: actions.changeCurrentMonday,
changeCurrentMonth: actions.changeCurrentMonth,
changeCurrentSchedule: actions.changeCurrentSchedule
}, dispatch);
}
export default connect(null, mapDispatchToProps)(Schedule);
const store = createStore(reducer,applyMiddleware(mid1,mid2));
再回头看看createStore方法，方法一共三个形参：reducer, preloadedState, enhancer
//
第二个参数是function且第三个参数不存在时，将二三参数互换
if (typeof preloadedState === ‘function‘ && typeof enhancer === ‘undefined‘) {
enhancer = preloadedState
preloadedState = undefined
}
//确保第三个参数enhancer是一个函数 如果是 用enhancer(createStore)(reducer, preloadedState)作为返回值if (typeof enhancer !== ‘undefined‘) {
if (typeof enhancer !== ‘function‘) {
thrownew Error(‘Expected the enhancer to be a function.‘)
}
return enhancer(createStore)(reducer, preloadedState)
}
所以上面的createStore执行的实际上是applyMiddleware(mid1,mid2)(createStore)(reducer, preloadedState)，applyMiddleware返回一个闭包函数，最终执行的代码为
function(reducer, preloadedState) { const store = createStore(reducer, preloadedState) let dispatch = () => { thrownew Error( `Dispatching while constructing your middleware is not allowed. ` + `Other middleware would not be applied to this dispatch.` ) } const middlewareAPI = { getState: store.getState, dispatch: (reducer, preloadedState) => dispatch(reducer, preloadedState) } const chain = middlewares.map(middleware => middleware(middlewareAPI)) dispatch = compose(...chain)(store.dispatch) return { ...store, dispatch } }
applyMiddleware是通过增强dispatch实现的，最终将增强后的dispatch返回，其中middlewareAPI 中的dispatch使用()=>dispatch()是为了保证每一个中间件的dispatch引用独立。
假如有两个中间件func1,func2（中间件格式({state,dispatch})=>(next)=>(action)=>{return next(aciton);}）
chain的格式为
[//func1
(next)=>(action)=>{next(action);},
//func2
(next)=>(action)=>{next(action);}]
现在看一下compose实现
export defaultfunction compose(...funcs) { if (funcs.length === 0) { return arg => arg } if (funcs.length === 1) { return funcs[0] } return funcs.reduce((a, b) => (...args) => a(b(...args))) }
redux-thunk原理
function
createThunkMiddleware(extraArgument) {
return ({ dispatch, getState }) => next => action => {
if (typeof action === ‘function‘) {
return action(dispatch, getState, extraArgument);
}
return next(action);
};
}
const thunk = createThunkMiddleware();
thunk.withExtraArgument = createThunkMiddleware;
export default thunk;
thunk异步就是通过不调用next(action)跳出循环实现的。
通过dispatch一个异步function(例func1)拦截更新state，在func1中再dispatch一个action实现异步更新state数据。
参考文章：
https://segmentfault.com/a/1190000004485808
原文：https://www.cnblogs.com/xcsun/p/9144787.html
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